Use CLIntegracon for Integration Tests

Inspired by the idea of the integration tests in CocoaPods, I wrote the gem [CLIntegracon](https://github.com/mrackwitz/CLIntegracon), which realizes a very similar concept, but usable for *any* CLI executable. As it keeps backwards-compatibility with the existing specs, there is no change required to them and it was easy to integrate it back.

It only even adds one requirement more, per executed command line: it looks for unexpected files, which are not configured as ignored and fails if there are any. All specs should pass.

The execution output has altered a tiny bit. (The printed command line doesn't carry the `$` anymore.) But this is currently ignored anyways and the specs didn't match them, because there were issues with different output formatting on Travis as it seems from the comments given and kept in `integration.rb`.
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